The invention relates generally to modulating clutches for motor vehicle drivelines and more specifically to a clutch having a biasing or preloading member which establishes a predetermined maximum torque throughput without activation of the clutch operator.
Controls systems and transfer cases having both electric and hydraulic modulating clutches have found broad application in adaptive vehicle drive systems. Such systems generally monitor the speeds of the front and rear drive shafts or compute such speeds by taking averages of individual readings of the two front and two rear wheels and, upon determining a speed difference between the drive shaft speeds or average speeds of the wheels, energize the modulating clutch according to a predetermined program to drive the speed difference and thus wheel slip back to zero.
Typically, such modulating clutches are physically disposed in a transfer case adjacent and driven by the output of the vehicle transmission and operably disposed between the primary and secondary drivelines. Systems exemplary of this configuration are disclosed in co-owned U.S. Pat. Nos. 5,407,024 granted Apr. 18, 1995 and 5,485,894 granted Jan. 23, 1996.
Typically, when such modulating clutches are inactive (during periods of equal drive shaft speed), they transfer either no torque from the primary driveline to the secondary driveline or a residual or minimal torque resulting merely from the internal drag of the clutch components. From a vehicle operational standpoint, such residual torque transfer is negligible and thus irrelevant. While such configurations provide exceptionally accurate torque modulation and thus prop shaft speed control when energized, they do have the drawback that until the clutch is activated or energized, substantially no torque transfer occurs. Stated somewhat differently, until or unless energy is expended to actuate the clutch, no torque transfer occurs. This is, of course, true whether the modulating clutch is disposed in a transfer case or a differential.
Given this operational rule, maintaining even a minimal, standby torque which maintains the secondary driveline components in a staged condition, that is, with all slack taken up, ready to transfer drive torque, can be accomplished only by the constant consumption of energy. In a clutch having an electromagnetic operator, this constant energization of the electromagnetic coil not only constantly consumes electrical energy but can also result in objectionable magnetization of driveline components and other proximate ferrous vehicle components which, while not deleterious, is preferably avoided.
From the foregoing, it is apparent that improvements relating to the art of modulating clutches specifically intended for use in the drivelines of motor vehicles addressing these drawbacks would be desirable.